1. Field of the Invention
This invention relates to electronic flash devices, and more particularly to an electronic flash device of the type in which the flash tube is supplied with electrical energy from a storage capacitor to produce flash light in synchronism with the discharge thereof.
2. Description of the Prior Art
It is known in the art to provide an electronic flash device in which permits all of the electrical energy stored in a storage capacitor to be discharged through a flash tube in making an exposure of photographic film. With such an electronic flash device, the flash tube produces a flash light in a period of time shorter than that during which the electrical energy is discharged therethrough, as depicted in FIG. 2, wherein A designates a curve showing a variation with time of the voltage applied on the flash tube, B designates a curve showing a variation with time of the flash light intensity as the evervarying voltages of curve A are applied to the flash tube, Va designates a voltage level at which the flash tube is stopped from, or actuated for, emitting flash light, and Vb designates a voltage level lower than Va and at which an ion increase phenomenon in the flash tube is caused to occur, the voltage Vb being hereinafter referred to as "discharge maintenance critical voltage". As a result, the flash tube is energized without production of flash light during a time interval from a time t.sub.1 with the voltage Va to a time t.sub.2 with the voltage Vb whenever electrical energy is discharged from the storage capacitor through the flash tube. Because of such characteristics of the flash tube, the conventional electronic flash device has the disadvantages of providing less flash light in relation to the amount of electrical energy used up, namely, operating at a lower luminous efficiency, and of requiring a longer time period for the next charging of the storage capacitor than that which would be otherwise required when the duration of discharge of the storage capacitor is terminated at the time t.sub.1, or in synchronism with the termination of flash light emission of the flash tube. In the latter connection, as shown in FIG. 3, the difference between time intervals necessary for a storage capacitor having a capacitance of 2000 .mu.F to be charged from zero voltage to a predetermined voltage corresponding to 100% storage of electrical energy and to be charged from a voltage corresponding to 10% storage of electrical energy to the predetermined voltage amounts to as long as 3 seconds. It will be appreciated that a good fraction of the charging period time is spent in vain for the next charging operation of the storage capacitor, since the conventional device permits discharge even after the time t.sub.1 or the termination of flash light production. Such disadvantages have been also encountered with the use of another type of electronic flash devices, as, for example, disclosed in U.S. Pat. No. 3,591,829, in which the flash tube is provided with a switching element connected in series therewith and arranged to be controlled by an output signal of a light measuring circuit responsive to the light reflected from an object being photographed, particularly when the object distance is relatively long.